Question: Which of these numbers is composite? ${2,\ 5,\ 23,\ 85,\ 89}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 2, 5, 23, and 89 each have only two factors. The factors of 85 are 1, 5, 17, and 85. Thus, 85 is the composite number.